The present invention relates to a control system for an electromagnetic clutch for an automatic transmission of a motor vehcle.
An automobile provided with a continuously variable belt-drive transmission with an electromagnetic clutch is disclosed in EP-A No. 151038. The electromagnetic clutch of the transmission is controlled by a control system to provide various operational modes such as a starting mode of a vehicle, reverse excitation mode, drag mode, and mode of lock-up engagement. One of the modes is selected in accordance with a position of a selector lever and driving conditions to control the electromagnetic clutch.
In the drag mode, a small clutch current flows in a coil of the electromagnetic clutch when an accelerator pedal is released in a low vehicle speed range thereby applying a small drag torque to a driven member of the clutch. Accordingly, at a start of the vehicle, the power of an engine is properly transmitted to driving wheels through the transmission, so that the vehicle can be smoothly started.
However, the drag torque is exerted on the engine to reduce idling speed thereof. Accordingly, the drag torque is set to such a small value as to prevent a large reduction of the idling speed. However, the idling speed may decrease at cold engine operation or in a state of incorrect idling adjustment. In such a particular idling state, the drag torque is too large to maintain the idling speed, causing decrease of the speed or stalling of the engine.